SIEMENS

Anybody visiting Jesper Møller at his favorite workplace needs to have a head for heights, good sea legs, and no inclination toward claustrophobia. Secured with ropes, we climb narrow ladders and ride unsteady freight elevators in order to get to the top of a windowless tower. On arrival, Møller invites his guests into the inner sanctum: the approximately 6 m-long cylinder that forms the head of a wind power plant.

A neon tube lights up the long shaft containing the gearbox, which transforms the rotation of the blades into a generator speed of 1,500 rpm. The generator is hidden at the back and can produce 2.3 MW of electrical power once the wind speed exceeds 11 m/s — but only if no visitors are present in the nacelle. "When anyone is visiting, the wind turbines are switched off for safety reasons," says Møller, who heads Offshore Technology at Siemens Wind Power division in Denmark. However, this is small consolation for visitors. Even though you are standing on a secure grid, you can’t help but feel there’s very little between you and the abyss beneath your feet. The North Sea swell is lapping at the foundations 60 m below. At the same time, the structure sways lightly in the wind — despite its weight of over 300 t. "It’s designed to do that," says Møller, "because flexibility is what provides our wind power plants with their tremendous stability. Even severe storms haven’t caused any problems."

Møller presses a switch and two roof wings open up above the nacelle to unveil a view of the North Sea. Dozens of wind turbines extend out in a row toward the horizon like a string of pearls. Some are rotating energetically in the breeze; others are waiting to be commissioned, while a few more are mere foundations protruding out of the sea. Horns Rev II is the name of this wind farm, which is situated on a sandbank about 30 km off the Danish coast. The park is still under construction but when completed in Fall 2009, it will be the largest offshore wind farm in the world. A total of 91 turbines from Siemens will then be able to pump around 210 MW of electrical power into the network — enough to supply over 136,000 households with electricity.

World Record for Wind Power. Such superlatives are nothing special by Denmark’s standards because they are already multiple world record holders. This small kingdom is not only the largest producer of wind power plants, but also generates 20 % of its energy requirements with wind power. In comparison, Germany, has so far only managed 7 %. Perhaps the figures aren’t so surprising when you consider that Denmark is a windy country and enjoys only ten calm days a year. On really windy days, the windmills can produce half of the country’s electricity, and on a stormy night, this figure can even rise to 100 %.

However, this bounty of green energy does have its downside. Because such plants rely on the wind, long-term energy production plans are out of the question. As a result, these white giants can play only a limited role when it comes to meeting the fluctuating demand for grid power. In contrast, other types of power plants, such as gas and cogeneration plants, can be run up or run down according to demand. That’s why Energinet.dk, the state-run network operator, uses a sophisticated energy management system that is partially based on several weather forecasting systems to get the best out of variable wind energy.

In order to quickly respond to fluctuations, excess wind-generated electricity is diverted to Norway’s pumped storage power plants to be used later during calm weather. Although currently capable of coping with peak loads and stabilizing the network, this arrangement may not be equal to future demands — particularly as the Danish government plans to substantially expand its use of wind power in coming years.

And that’s just fine as far as Møller is concerned. He has been building wind farms for the last ten years and has developed a special bond with his turbines. "Although the work is routine," he says. "I experience something special every time I ascend a windmill and look out over the North Sea." Just in front of him, the huge 45-m rotor blades stretch into the sky, their tips roaring through the air at 220 km/h and producing enough energy to boil six liters of water every second. Depending on the strength of the wind, it’s possible to alter the white blades’ angle of attack so that they operate in the most efficient manner.

The 82 t-nacelle can also turn on its own axis in the wind — courtesy of a computer-controlled system. A host of sensors, both inside and outside the compartment, continuously measure the vibrations of the machine parts. Using this data, experts from Siemens can remotely recognize when a problem is brewing, because each unusual reading triggers an alarm. In this way experts can detect anomalies and prevent damage from occurring.

Only the most observant visitors notice that the nacelle and blades incline slightly upwards at an angle of 7 °. "We have to maintain a safe distance between the blades and the mast," says Møller. "They are so flexible that they bend inward considerably in stormy conditions."

Robust Blades. Søren Kringelholt Nielsen and his 800 employees at Siemens Rotor Blade Manufacturing, which is located 230 km away in Aalborg, ensure that the huge blades are flexible. All the blades for the European market are produced here. The floor of the factory is covered with neat rows of the gigantic rotor blades, each of which is bigger than the wing of a jumbo jet. The surface of the blades is so smooth that you can’t see or feel a single seam, while the edges at the tips are nearly as sharp as knives. Despite their size, the aerodynamic blades can be bent by several centimeters using nothing more than your hand.

"This apparent fragility is deceiving," says Nielsen, who heads Rotor Blade Manufacturing in Aalborg. "The blades are extremely robust. Imagine placing a mid-sized car at the end of a 3-km beam. The forces that are being placed on the other end of the beam are the same as those a rotor blade needs to withstand during strong winds," explains Nielsen.

The secret of the blades’ stability can be found in the 250-m-long production hall where they are manufactured using "Integral Blade Technology," a patented process (see Pictures of the Future, Fall 2007, "Catching the Wind"). What’s remarkable is that the rotor blades are manufactured as a single component without seams — a method that only Siemens has mastered. At the start of the process, workers roll out long alternate layers of fiberglass mats and balsa wood in a form to make a kind of "sandwich." The bottom and top sections are subsequently joined and a vacuum is created inside. The vacuum sucks liquid epoxy resin through the fiberglass mats and the balsa wood. Here, the resin finds its way through all of the layers and evenly joins the two sides of the blade. Finally, the blades are "baked" in a gigantic oven at a temperature of 70 °C for eight hours. "At the end of this process we have a seamless rotor blade with no weak points," says Nielsen. Weaknesses are unacceptable because maintenance costs must be kept to a minimum during the 20 years in which the blades must withstand wind and weather. "Repairs on the open sea cost about ten times as much as repairs on land," says Nielsen. To further increase their resilience, all the blades are equipped with a lightning conductor. "Statistically, each blade will be struck at least once by lightning."

Swimming Packhorse. By the time a blade begins its life on a mast at Horn Rev II, it will have an amazing journey behind it. First of all, blades are strapped onto articulated trucks for the 280-km journey to Esbjerg harbor, one of Siemens’ transport hubs for wind farms in Europe. Here, the individual blades are attached to rotors and loaded — together with the nacelles and the masts — onto the "Sea Power," an assembly ship that transports the components of three separate wind power plants to their destinations in the North Sea. Gigantic cranes lift the 60-t rotors onto the deck of the ship, stacking three huge propellers per rotor on top of one another, before placing the tower sections and the nacelle beside them. This swimming packhorse then transports its freight, which weighs over 1,000 t, 50 km to Horns Rev II.

From his nacelle 60 m above the North Sea, Møller has spotted the Sea Power. "It takes six to eight hours to completely assemble a wind power plant," he says. The assembly ship’s crane lifts the steel tower, the nacelle, and finally the rotor onto a yellow pedestal — a steel foundation that was driven 20 m into the sandy seabed some time earlier. The components are then bolted together by hand.

"Naturally, this is possible only with good weather. As soon as the height of the waves exceeds 1.5 m the work is called off. And this can happen quite often on the North Sea, which is renowned for being rough," says Møller. He points at an old ferry that is anchored not far from the wind farm. "That’s our hotel ship. It’s home for the workers who are responsible for the installation and cabling of the wind mills. They spend two weeks at a time here at sea."

In contrast, stays in the nacelles, which are far from comfortable, are of course much shorter. The limit is three days. In case evacuation is impossible in the face of a rapidly-developing storm, each tower is outfitted with emergency storage facilties for fresh water and energy bars. On the other hand, there are visitors who have climbed the tower with Jesper Møller who have indicated that they would rather stay a little longer because, even when there is no emergency, the cramped nacelle seems preferable to the idea of climbing back down to a swaying boat at the foot of the mast — especially when you’ve forgotten your seasickness pills.